Although the detailed pathway of carbon, by which carbohydrates are formed from carbon dioxide during photosynthesis, has been established, comparatively little is known about the participation and transport of hydrogen in the complex series of reactions now known to be involved in photosynthesis. The transport of hydrogen may occur concurrently with the redox reactions of photosynthetic electron transport leading to the formation of ATP, as well as in the reduction of carbon dioxide. The participation of the chlorophylls, the carotenoids, the quinones, and other lipoid components in photosynthetic hydrogen transport has been the subject of considerable speculation. In an attempt to elucidate the pathway of hydrogen transport, we are studying the tritium labeling pattern in the lipid extracts of Rhodospirillum rubrum after the bacteria have been illuminated in growth medium containing tritiated water.